Is the DNS server on the Zimbra server or elsewhere in your LAN and is this a single server or what? My inclination would be an intermittent problem with either DNS or perhaps your NIC. What has EFW got to do with this? Aren't your Zimbra & DNS servers behind EFW?

Just for confirmation you might as well post the output of the following commands:

This problem does not happen when using a client such as Thuderbird or Outlook, or it looks like it doesn't happen. It appears more like an issue with the web client - but I am now clutching at straws!

The other information you've posted seems OK but you missed the hosts file, could you post that as well? Is there anything else running on this server? Is it a single server? How much RAM does it have? Have you monitored the server performance during the period you've had this problem? Have you done any updates to CentOS recently that might have caused any problems? Is Zimbra on a HD that's local to the server or and a SAN or network mount?

The Zimbra box also handles Samba, there's no integration between that and Zimbra.

The server itself has 2GB RAM and is a Dell Poweredge T605

There's two SATA disks in there with a single logical 280GB drive with 106GB available.

When monitoring cpu/memory usage with 'top' cpu% reaches 10% when an email is sent (amavisd) and java hits 21% memory. When the error sending appears, there's nothing out of the ordinary there. It's as if the web client can't 'see' the rest of the system.

There have been no updates to CentOS (if it ain't broke don't fix it!)